linux-kernel - Re: [PATCH 08/10] spi: aspeed: Calibrate read timings

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Date:   Fri, 25 Feb 2022 14:48:09 +0530
From:   Pratyush Yadav <p.yadav@...com>
To:     Cédric Le Goater <clg@...d.org>
CC:     <linux-spi@...r.kernel.org>, <linux-mtd@...ts.infradead.org>,
        Mark Brown <broonie@...nel.org>,
        Tudor Ambarus <tudor.ambarus@...rochip.com>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Richard Weinberger <richard@....at>,
        Vignesh Raghavendra <vigneshr@...com>,
        <linux-aspeed@...ts.ozlabs.org>, Joel Stanley <joel@....id.au>,
        Andrew Jeffery <andrew@...id.au>,
        Chin-Ting Kuo <chin-ting_kuo@...eedtech.com>,
        <devicetree@...r.kernel.org>, Rob Herring <robh+dt@...nel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 08/10] spi: aspeed: Calibrate read timings

On 14/02/22 10:42AM, Cédric Le Goater wrote:
> To accommodate the different response time of SPI transfers on different
> boards and different SPI NOR devices, the Aspeed controllers provide a
> set of Read Timing Compensation registers to tune the timing delays
> depending on the frequency being used. The AST2600 SoC has one of
> these registers per device. On the AST2500 and AST2400 SoCs, the
> timing register is shared by all devices which is a bit problematic to
> get good results other than for one device.
> 
> The algorithm first reads a golden buffer at low speed and then performs
> reads with different clocks and delay cycle settings to find a breaking
> point. This selects a default good frequency for the CEx control register.
> The current settings are bit optimistic as we pick the first delay giving
> good results. A safer approach would be to determine an interval and
> choose the middle value.
> 
> Due to the lack of API, calibration is performed when the direct mapping
> for reads is created.

The dirmap_create mapping says nothing about _when_ it should be called. 
So there is no guarantee that it will only be called after the flash is 
fully initialized. I suggest you either make this a requirement of the 
API, or create a new API that guarantees it will only be called after 
the flash is initialized, like [0].

[0] https://patchwork.ozlabs.org/project/linux-mtd/patch/20210311191216.7363-2-p.yadav@ti.com/

> 
> Cc: Pratyush Yadav <p.yadav@...com>
> Signed-off-by: Cédric Le Goater <clg@...d.org>
> ---

-- 
Regards,
Pratyush Yadav
Texas Instruments Inc.